[0001] The present invention relates to an aqueous primer composition for a fluororesin
coating, that is, a primer composition which is used to adhere the fluororesin to
various metal substrates when the fluororesin is coated on such substrates.
[0002] When fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene
copolymers (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA),
etc. are coated on various metal substrates, it is difficult to apply the fluororesins
directly on the metal substrates since the fluororesins are non-tacky as their nature,
[0003] Therefore, to coat the fluororesins on the substrates, primer compositions, which
have adhesion properties to the substrates and also to the fluororesin coating to
be applied on the primers, have been developed and practically used.
[0004] In recent years, engineering plastics having excellent heat resistance are developed
and thus new primers for a fluororesin coating are being developed by combining such
plastics and the fluororesins.
[0005] For example, JP-A-61-111352 discloses a fluororesin coating composition consisting
of an aqueous dispersion of polyethersulfone and a fluororesin. However, when this
composition is used as a primer, the corrosion resistance, which is the drawback of
polyethersulfone, tends to further deteriorate.
[0006] JP-A-59-199774 discloses a primer composition comprising polyamideimide or polyimide
as a binder component, and a specific fluororesin. However, this composition is severely
discolored in the course of baking (calcination) so that the color is changed to dark
brawn. Thus, the color tone of the coating is limited. In addition, since the composition
contains the highly water-absorbing binder resin, the coating of the fluororesin tends
to blister due to low steam resistance in the case of rice cookers, in which rice
is quickly heated, such as IH cooking jars, when the primer composition is used as
the primer for a PFA powder coating which is increasingly used as the coating of the
rice cookers.
[0007] JP-B-2702041 discloses a primer composition comprising polyethersulfone, polyamideimide
and/or polyimide, a fluororesin, and metal powder, which are dissolved or dispersed
in an organic solvent. However, in the primer composition disclosed in this patent,
the dispersion medium is the organic solvent such as N-methylpyrrolidone, and the
binder resin is dissolved in the solvent. Therefore, it is difficult to increase the
solid content of the coating. In addition, the composition of the dispersion medium
has to be improved from the viewpoint of the environmental protection, which is one
of the big social problems.
[0008] EP 0 389 966 discloses a fluororesin-coated article comprising an aluminum or aluminum
alloy base having a surface which has been finely roughened by electrochemical etching
or chemical etching, which comprises a first fluororesin layer provided on the roughened
surface of the base as well as a second fluororesin layer provided on the first fluororesin
layer.
[0009] US 5,230,961 discloses an improved non-stick coating system that can be applied to
untreated smooth substrates with a primer of polytetrafluoroethylene and perfluorinated
copolymer of tetrafluoroethylene and hexafluoropropylene to give a concentration gradient.
[0010] WO 92/10549 teaches an improved non-stick coating system that can be applied to untreated
smooth substrates with a primer of a first polytetrafluoroethylene having a specified
melt viscosity plus a second polytetrafluoroethylene having a specified melt viscosity
with the melt viscosity of the first being higher than that of the second to give
a concentration gradient.
[0011] One object of the present invention is to provide a primer composition for a fluororesin
coating, which has good adhesion properties to substrates, and improved corrosion
resistance and steam resistance, causes no environmental problems, and has good workability.
[0012] The present inventors have found that, when polyethersulfone having good steam resistance
but low corrosion resistance as a plastic component contributing to the adhesion to
the substrate, and polyamideimide and/or polyimide having good corrosion resistance
but low steam resistance are mixed in the form of particles at a ratio, which can
make full use of the characteristics of the both resins, in an aqueous medium together
with a fluororesin and the dispersion is formed in the form of a film, the characteristics
of the two resins having low compatibility each other can be exhibited, and the above-described
problems of the conventional primer compositions for the fluororesin coating can be
solved.
[0013] In addition, since the composition is in the form of an aqueous dispersion, the resin
components are not dissolved in the solvent unlike the solvent base primer compositions,
and the high solid content can be achieved. Thus, the composition of the present invention
can greatly contribute to the resolution of the environmental problem which is one
of the big social problems, and improve the easiness and safety of the coating works
because of the use of the aqueous medium as the dispersion medium and the increase
of the solid content.
[0014] Accordingly, the present invention provides an aqueous primer composition for a fluororesin
coating comprising a fluororesin, polyethersulfone, and at least one resin selected
from the group consisting of polyamideimide and polyimide, which are all dispersed
in water, wherein a weight ratio of polyethersulfone to polyamideimide and/or polyimide
is from 85:15 to 65:35, and a weight ratio of the total amount of polyethersulfone
and polyamideimide and/or polyimide to the fluororesin is from 15:85 to 35:65.
[0015] In the present invention, the weight ratio of polyethersulfone to polyamideimide
and/or polyimide is from 85:15 to 65:35, preferably from 80:20 to 65:35. When the
amount of polyethersulfone exceeds the above range, the corrosion resistance, which
is one of the important properties of the primer for the fluororesin coating, greatly
deteriorates. When the amount of polyamideimide and/or polyimide exceeds the above
range, the freedom of the color tone of the coating is limited because of the discoloration
in the course of baking. In addition, the steam resistance decreases when the coating
of meltable fluororesins such as PFA powder is applied on the primer.
[0016] In the present invention, the weight ratio of the total amount of polyethersulfone
and polyamideimide and/or polyimide to the fluororesin is from 15:85 to 35:65, preferably
from 20:80 to 35:65. When the amount of the fluororesin exceeds the above range, the
adhesion of the primer to the substrates decreases, and thus the corrosion resistance
deteriorates. When the amount of the fluororesin is smaller than the above range,
the adhesion of the coating of the fluororesin as a top coating to the primer decreases
so that the layers may be peeled, and the steam resistance and the corrosion resistance
deteriorate to cause the blisters between the layers.
[0017] The concentration of the solids in the primer composition of the present invention
is not limited. However, such a concentration is usually 50 wt. % or less, preferably
40 wt. % or less, from the viewpoint of the handling of the composition in the transportation
and application of the composition.
[0018] The kind of the fluororesin contained in the primer composition of the present invention
is not limited, and any fluororesin coating, that is used in the conventional primers
for the fluororesin, may be used. Examples of the fluororesin include polytetrafluoroethylene
(PTFE), modified PTFE, tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkyl
vinyl ether copolymers (PFA), etc. In addition, fluororesins comprising resin particles
with a core-shell structure may be used. The fluororesins may be used independently
or in admixture of two or more.
[0019] In the present invention, polyethersulfone, polyamideimide and/or polyimide as the
binders are pulverized and stabilized in the dispersed state in water. When the binder
resins have too large particle sizes, the dispersion stability tends to deteriorate
and the resin particles tend to precipitate and further the surface smoothness of
the coated film of the primer decreases. In addition, when the particle size is large,
the properties of each binder resin prior to blending remain, so that the intended
properties of the coating film may not be attained. Thus, the average particle size
of polyethersulfone and polyamideimide or polyimide is usually 8 µm or less, preferably
5 µm or less.
[0020] When the particle size of the binder resins is small, the thixotropy of the composition
increases and also the viscosity of the composition increases, so that the handing
and application of the composition become difficult, although the surface smoothness
is satisfactory. Therefore, a suitable average particle size is from 0.5 µm to 8 µm.
[0021] In addition to the binder resins and the fluororesin, the primer composition of the
present invention may contain conventional additives which are contained in the aqueous
primers for the fluororesin coating.
[0022] For example, pigments may be compounded for coloring. As the pigments, inorganic
pigments and carbon back, which are stable at high temperature, are mainly used. Preferably,
the amount of the pigments does not exceed about 20 wt. % of the resin components,
since the addition of the pigments may decrease the corrosion resistance.
[0023] Metal powder having brightness such as aluminum powder, stainless steel powder, etc.
can be compounded in the primer composition of the present invention. Since polyethersulfone
has good transparency, the primer composition of the present invention comprising
polyethersulfone can provide a film with brightness by the addition of the metal powder,
even when the top coating is a clear coating. The amount of the metal powder added
is preferably 10 wt. % or less of the resin components, since the addition of the
metal powder may decrease the corrosion resistance of the primer.
[0024] It is also preferable to add a surfactant as an additive to improve the dispersion
stability of the resins. Examples of the surfactant include nonionic surfactants and
anionic surfactants. The amount of the surfactant added is preferably from 1 to 15
wt. % of the solids in the primer composition.
[0025] The viscosity of the primer composition of the present invention may be adjusted
by the addition of a water-soluble polymer to improve the workability in the application
process and maintain the storage stability of the composition. However, when the water-soluble
polymer is thermally decomposable, the decomposed materials may deteriorate the corrosion
resistance. As the water-soluble polymer which can be used in the present invention,
polyvinyl alcohol, polyvinylpyrrolidone, methylcellulose, hydroxyethylcellulose, etc.
can be exemplified, although other water-soluble polymers may be used.
[0026] Furthermore, the composition of the present invention may optionally contain conventional
additives of primer coatings such as a leveling agent, a defoaming agent, an anti-settling
agent, etc.
[0027] The dispersion medium of the primer composition of the present invention is water,
although an organic polar solvent such as N-methyl-2-pyrrolidone (NMP), dimethylacetamide,
dimethylformamide, etc. may be added to the composition to facilitate the film formation
after the application of the composition.
[0028] The amount of the polar solvent added depends on its kind. When the amount of the
polar solvent is too low, the effect to facilitate the film formation may be insufficient,
while when the amount is too high, the solvent will dissolve the particle surface
of polyethersulfone, polyamideimide or polyimide dispersed and thus the stability
of the composition deteriorates.
[0029] The primer composition of the present invention may be prepared as follows:
[0030] Firstly, polyethersulfone and water are charged in a suitable pulverizing machine
such as a ball mill and pulverized until the particle size of polyethersulfone reaches
the above-described preferable particle size. In this step, the organic polar solvent
may be added, if desired.
[0031] Polyamideimide and/or polyimide can be pulverized in the same manner as above. When
polyamideimide and/or polyimide are dissolved in the solvent, water is added to the
solution to precipitate the resins, and then the precipitated resins are pulverized
with a suitable pulverizing machine such as a ball mill until the particle size of
polyamideimide and/or polyimide reaches the above-described particle size.
[0032] Next, the aqueous dispersion of polyethersulfone and that of polyamideimide and/or
polyimide are mixed with the dispersion of the fluororesin to obtain the primer composition
of the present invention.
[0033] The primer composition of the present invention can be applied to the substrate,
dried and baked by the same methods as those used to coat the conventional aqueous
primer compositions, and thus the primer is formed on the surface of the substrate.
[0034] When the primer composition of the present invention is used, the kind of the fluororesin
used as a top coating formed on the primer is not limited. That is, the primer formed
from the primer composition of the present invention may be used as a primer for a
top coating of PTFE, PFA or EFP dispersion, a PFA or FEP powder coating, and a fluororesin
film.
[0035] The primer composition for the fluororesin coating according to the present invention
has good workability and is friendly to the environment. Furthermore, the fluororesin
coating film formed on the primer layer which is formed from the aqueous primer composition
of the present invention has good steam resistance and corrosion resistance, which
are required for rice cookers, grill pans, etc.
[0036] The present invention will be illustrated by the following Examples, which do not
limit the scope of the invention in any way. In the Examples, "%" and "parts" are
by weight.
Preparation of aqueous dispersion of polyethersulfone
[0037] Polyethersulfone (hereinafter referred to as "PES") 5003P (manufactured by ICI) (60
g) and ion-exchanged water (60 g) were milled with a ceramic ball mill until the PES
particles were completely pulverized (about 10 minutes). Then, N-methyl-2-pyrrolidone
(hereinafter referred to as "NMP") (180 g) was added, and the mixture was further
milled for 48 hours.
[0038] The dispersion obtained was pulverized with a sand mill by seven passes to obtain
the aqueous dispersion having a PES concentration of about 20 %. PES in the dispersion
had a particle size of 2 to 3 µm. The obtained dispersion will be referred to as "Aqueous
PES Dispersion". Aqueous PES Dispersion was used in the preparation of the compositions
in Examples and Comparative Examples.
Preparation of aqueous dispersion of polyamideimide
[0039] A commercially available solution of polyamideimide (hereinafter referred to as "PAI")
(a NMP solution having a solid content of 29 %) was poured in water to precipitate
the resin. Then, the resin was pulverized with a ball mill for a suitable time to
obtain a dispersion of the resin having an average particle size of 2 µm. The dispersion
obtained had a solid content of 20 %. This dispersion will be referred to as "Aqueous
PAI Dispersion". Aqueous PAI Dispersion was used in the preparation of the compositions
in Examples and Comparative Examples.
Example 1
[0040] Aqueous PES Dispersion and Aqueous PAI Dispersion were mixed such that the weight
ratio of PES to PAI (solids) was 85:15. Then, the PTFE dispersion (solid content:
60 %) was added to the mixture such that the weight ratio of the total solid amount
of PAI and PES to the fluororesin (PES+PAI:fluororesin) was 20:80, and furthermore
polyvinyl alcohol as a thickening agent was added to adjust the viscosity at 135 centipoise/0.135
Pa·s (Brookfield viscometer; No. 2 roter; 60 rpm) . Thus, an aqueous dispersion having
a solid content of 34 % was obtained.
Example 2
[0041] Aqueous PES Dispersion and Aqueous PAI Dispersion were mixed such that the weight
ratio of PES to PAI (solids) was 75:25. Then, the PTFE dispersion (solid content:
60 %) was added to the mixture such that the weight ratio of the total solid amount
of PAI and PES to the fluororesin was 20:80, and furthermore polyvinyl alcohol as
a thickening agent was added to adjust the viscosity at 130 centipoise/0.13 Pa·s (Brookfield
viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion having a solid content
of 34 % was obtained.
Example 3
[0042] Aqueous PES Dispersion and Aqueous PAI Dispersion were mixed such that the weight
ratio of PES to PAI (solids) was 65:35. Then, the PTFE dispersion (solid content:
60 %) was added to the mixture such that the weight ratio of the total solid amount
of PAI and PES to the fluororesin was 20:80, and furthermore polyvinyl alcohol as
a thickening agent was added to adjust the viscosity at 120 centipoise/0.12 Pa·s (Brookfield
viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion having a solid content
of 30 % was obtained.
Example 4
[0043] Aqueous PES Dispersion and Aqueous PAI Dispersion were mixed such that the weight
ratio of PES to PAI (solids) was 75:25. Then, the PTFE dispersion (solid content:
60 %) was added to the mixture such that the weight ratio of the total solid amount
of PAI and PES to the fluororesin was 15:85, and furthermore methylcellulose as a
thickening agent was added to adjust the viscosity at 130 centipoise/0.13 Pa·s (Brookfield
viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion having a solid content
of 37 % was obtained.
Example 5
[0044] Aqueous PES Dispersion and Aqueous PAI Dispersion were mixed such that the weight
ratio of PES to PAI (solids) was 75:25. Then, the PTFE dispersion (solid content:
60 %) was added to the mixture such that the weight ratio of the total solid amount
of PAI and PES to the fluororesin was 25:75, and furthermore methylcellulose as a
thickening agent was added to adjust the viscosity at 135 centipoise/0.135 Pa·s (Brookfield
viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion having a solid content
of 34 % was obtained.
Example 6
[0045] Aqueous PES Dispersion and Aqueous PAI Dispersion were mixed such that the weight
ratio of PES to PAI (solids) was 75:25. Then, the PTFE dispersion (solid content:
60 %) was added to the mixture such that the weight ratio of the total solid amount
of PAI and PES to the fluororesin was 35: 65, and furthermore methylcellulose as a
thickening agent was added to adjust the viscosity at 130 centipoise 0.13 Pa·s (Brookfield
viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion having a solid content
of 34 % was obtained.
Comparative Example 1
[0046] The PTFE dispersion (solid content: 60 %) was added to Aqueous PES Dispersion such
that the weight ratio of PES to PTFE (solids) was 20:80, and furthermore polyvinyl
alcohol as a thickening agent was added to adjust the viscosity at 130 centipoise
0.13 Pa·s Brookfield viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion
having a solid content of 34 % was obtained.
Comparative Example 2
[0047] The PTFE dispersion (solid content: 60 %) was added to Aqueous PAI Dispersion such
that the weight ratio of PAI to PTFE (solids) was 20:80, and furthermore polyvinyl
alcohol as a thickening agent was added to adjust the viscosity at 140 centipoise
0.14 Pa·s (Brookfield viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion
having a solid content of 34 % was obtained.
Comparative Example 3
[0048] Aqueous PES Dispersion and Aqueous PAI Dispersion were mixed such that the weight
ratio of PES to PAI (solids) was 50:50. Then, the PTFE dispersion (solid content:
60 %) was added to the mixture such that the weight ratio of the total solid amount
of PAI and PES to the fluororesin was 20:80, and furthermore polyvinyl alcohol as
a thickening agent was added to adjust the viscosity at 135 centipoise/0.135 Pa·s
(Brookfield viscometer; No. 2 roter; 60 rpm). Thus, an aqueous dispersion having a
solid content of 34 % was obtained.
Comparative Example 4
[0049] PES 5003P was dissolved in NMP to obtain a solution having a solid content of 20
%. To this solution, NMP and a fluororesin (FEP powder) were added such that the weight
ratio of PES to the fluororesin (solids) was 20:80. Then, the mixture was dispersed
in a ball mill to obtain a dispersion having a solid content of 18 %. The obtained
dispersion had a viscosity of 180 centipoise/0.18 Pa·s (Brookfield viscometer; No.
2 roter; 60 rpm).
Comparative Example 5
[0050] PES 5003P was dissolved in NMP to obtain a solution having a solid content of 20
%. To this solution, a solution of polyamideimide in NMP was added such that the weight
ratio of
PES to PAI was 75:25, and then NMP and a fluororesin (FEP powder) were added such that the weight
ratio of the total amount of PAI and PES to the fluororesin (solids) was 20:80. The
mixture was dispersed in a ball mill to obtain a dispersion having a solid content
of 18 %. The dispersion obtained had a viscosity of 165/0.165 Pa·s centipoise (Brookfield
viscometer; No. 2 roter; 60 rpm).
Production of a test plate:
[0051] A coated test plate was produced by the following procedures using each of the dispersions
prepared in Examples and Comparative Examples as a primer:
[0052] The surface of a pure aluminum plate (A-1050P) as a substrate was degreased with
acetone and roughened by sand blasting so that the surface roughness Ra was in the
range between 2.5 µm and 3.5 µm. The dusts on the surface were removed by air blowing,
and then each dispersion was applied such that the dry film thickness was 10 µm.
[0053] Then, the coated aluminum plate was dried with IR at about 100°C. Thereafter, a PFA
powder coating (ACX-31) was applied as a top coating and baked at 380°C for 20 minutes.
The thickness of the top coating was 40 µm.
Test methods:
[0054] The properties of the coated films were measured or evaluated as follows:
Resistance to Corrosion with ODEN premix
[0055] The surface of the coated film of the test plate was cross cut with a cutter knife
to form flaws reaching the substrate surface. The flawed test plate was dipped in
the solution of an
ODEN premix (manufactured by SB Foods) (20 g) dissolved in water (1 liter) and heated
at 70°C. Then, every 100 hours, the surface was checked whether any irregularities
such as blisters appear. The time in which the normal coating condition was maintained
is reported in Table 1.
Steam resistance test
[0056] The test plate was placed in steam under 6 atm. for 8 hours. After being removed
from the steam, the test plate was heated at 250°C for 2 minutes. With the above procedure
being one cycle, 20 cycles were repeated, and the formation of blisters was observed.
[0057] When no blister appeared after 20 cycles, the plate was ranked "PASS". When blisters
appeared, the plate was ranked "FAIL". The results are reported in Table 1 together
with the number of cycles when the blisters appeared.